During the early part of this fiscal year, we concentrated our effort in detecting and analyzing thermal and mechanical changes in the amphibian dermal glands produced by sympathetic nerve stimulation. We fabricated heat sensors designed to match the area of the skin under study and succeeded in recording the heat associated with noradrenaline release from the nerve terminals. Our finding that the onset of this heat precedes the beginning of the mechanical and electrical responses to sympathetic nerve stimulation is of special significance in the study of the action of noradrenaline on the dermal glands. In a meeting of physical chemists and chemical engineers held at this campus on December 10, 1989, an interesting piece of information was reported. Polymer gels with negative fixed charges were shown to undergo phase transitions accompanying reversible, discontinuous volume changes. Since we were in search of a solid physicochemical basis for explaining the nature of the rapid mechanical changes in nervous tissues, we began an investigation of the phenomenon of swelling in synthetic polymer gels. We found that poly (acrylic acid) gels immersed in salt solutions containing both univalent cations (e.g., sodium or potassium) and divalent cations (e.g., calcium) exist in one of the two stable states, either swollen or compact (shrunk). We also measured heat production associated with these discontinuous volume transitions. We now believe that similar discontinuous transitions take place in the superficial layer of nerve cells and fibers and that these transitions correspond to the beginning and end of an action potential.